According to an industrial metrology standard which has existed for a long time, electrical signals which are output by sensors and which are generated by the sensors on the basis of a physical or chemical law are converted into a measurement current, corresponding to the measurement range of a sensor, of 4 mA to 20 mA. The sensor is often augmented by a conversion stage, which is provided for this and is structurally combined with the sensor, and forms a sensor arrangement.
The measurement current can, on the one hand, be conducted via a transmission line, for example via a two-wire line, to a suitable display location, for example to a central measurement control station. On the other hand, power is supplied to the sensor arrangement via the transmission line.
In the latter context, DE-A 27 01 184 describes a circuit arrangement which serves for transmitting a measurement current, which is output by a sensor arrangement at a measurement location, to a current measuring resistor, which is situated at a distance therefrom, in conjunction with simultaneous power supply of the sensor arrangement via the current measuring resistor, which is connected in series with a power source, by means of a two-wire line.
Furthermore, DE-A 35 26 997 describes a circuit arrangement which serves for transmitting a measurement-current of 0 mA to 20 mA generated in a first circuit to a second circuit by means of a transformer, which belongs to a DC converter used for the power supply of the first circuit and situated on the side of the latter, the transformer having, on the side of the second circuit, two series-connected partial windings, to whose junction point a first pole of a power source is connected and to whose terminals which are remote from the junction point a second pole of the power source is connected.
Finally, DE-A 38 12 861 describes a circuit arrangement for intrinsically safe power supply from a non-intrinsically safe area by means of a DC converter situated in this area, to which DC converter a push-pull transformer belongs, which has a primary winding, which faces the non-intrinsically safe area and has two partial windings connected in series in opposite senses, and a secondary winding, which faces the intrinsically safe area and has two identical partial windings connected in series in opposite senses.